The multiple input multiple output (MIMO) technology is extensively applied in wireless communications systems to increase system capacities and ensure cell coverage. For example, in a Long Term Evolution LTE) system, transmit diversity based on multiple antennas, open-loop or closed-loop spatial multiplexing, and multi-stream transmission based on a demodulation reference signal (DM-RS) are used in a downlink. Among them, the DM-RS based multi-stream transmission is a main transmission mode in an LTE-Advanced (LTE-A) system and later systems.
In a conventional cellular system, a beam at a transmit side of a base station can be adjusted only in a horizontal dimension. In a vertical dimension, however, a fixed downtilt is used for every user. Therefore, various beamforming or precoding technologies or the like are all based on channel information in the horizontal dimension. In practice, however, because a channel is three-dimensional (3D), the fixed downtilt method cannot always optimize a system throughput. Therefore, a beam adjustment in the vertical dimension is of great significance to system performance enhancement.
A conception of a 3D beamforming technology is mainly as follows: A 3D beamforming weighted vector at an active antenna side is adjusted according to 3D channel information estimated at a user side, so that a main lobe of a beam in a 3D space “aims at” a target user. In this way, received signal power is increased greatly, a signal to interference plus noise ratio is increased, and further, the throughput of the entire system is enhanced. Schematic diagrams of comparison between a dynamic downtilt in 3D beamforming and a fixed downtilt of a conventional antenna are shown in FIG. 1 and FIG. 2. An antenna port model with a fixed downtilt is shown in FIG. 1, where corresponding to conventional 2D MIMO, a fixed downtilt is used for all users. An antenna port model with a dynamic downtilt is shown in FIG. 2, where for each physical resource block (PRB), a base station may dynamically adjust a downtilt according to a location of a served user. The 3D beamforming technology needs to be based on an active antenna system. Compared with a conventional antenna, the active antenna AAS further provides a degree of freedom in a vertical direction. FIG. 3 shows a schematic diagram of AAS antennas. It can be seen that there are multiple antennas in the vertical direction of AAS antennas. Therefore, a beam can be formed in the vertical direction dynamically, and a degree of freedom of beamforming in the vertical direction is added. FIG. 4 shows a flowchart in which data is processed in baseband and radio frequency networks, and transmitted through an AAS antenna. In a baseband processing part, a data stream at each layer undergoes precoding processing, and then is mapped to NP ports. After undergoing inverse fast Fourier transform (IFFT) and parallel-to-serial conversion, a data stream on each port enters a drive network in a radio frequency part, and then is transmitted through an antenna. Each drive network is a 1-to-M drive network, that is, one port corresponds to M antenna elements. FIG. 5 shows a schematic diagram of downtilt grouping. In the example, there are eight antenna ports, and each port drives four antenna elements to form a downtilt. In addition, four antenna ports (ports 0 to 3) in a horizontal direction have a same weighted vector in drive networks, and all point to a downtilt 0; the other four antenna ports (ports 4 to 7) have a same weighted vector, and all point to a downtilt 1.
In the prior art, spatially multiplexed multi-stream data can be transmitted only in a plane with a fixed downtilt by using a horizontal beam, and characteristics of a vertical space cannot be used to multiplex multiple data streams.